For some applications where vinyl chloride polymers are used in the form of plastisols, e.g., coating, there is a known demand for polymers in the form of a powder appropriate to the preparation of plastisols containing very small quantities of plasticizer, down to 25 parts by weight of plasticizer per 100 parts of polymer, and having the lowest possible viscosity both at low and high speed gradient.
Homo and co-polymers of vinyl chloride appropriate to give plastisols are prepared by homopolymerizing vinyl chloride in emulsion of microsuspension or by co-polymerizing vinyl chloride in emulsion or microsuspension with at least one other co-polymerizable monomer. On an industrial scale, the homo or co-polymerization provides a latex which generally contains from 40 to 55% by weight of dry material. To obtain the polymer in a powdered state, the latex is generally treated by drying and particularly by spray drying or flaking. The greater the weight of dry material in the latex, the less onerous is the drying operation. It is therefore desirable to start with a latex containing the largest possible weight of dry material. For polymerization on the other hand, the higher the weight of dry material in the latex to be prepared, the more emulsifying agent is required. Latices containing a large weight of dry material thus contain large quantities of emulsifying agent. After the drying process, these remain mixed with the polymer and are detrimental to its properties, such as the transparency of the articles obtained, its thermal stability, its reaction to light and ultra violet rays and the ability of bubbles to escape from the plastisols. In addition, the heat liberated during the polymerization reaction is more difficult to dissipate if the weight of water in the polymerizing medium is low, i.e., if the weight of dry material in the latex to be prepared is high. What is desired therefore is to prepare a latex containing a relatively low weight of dry material and, before carrying out the drying operation, to concentrate it by eliminating part of the aqueous phase in the liquid state.
In the co-pending application Ser. No. 914,352, filed June 12, 1978, now U.S. Pat. No. 4,245,070 issued Jan. 13, 1981 description is made of a method of preparing homo and co-polymers of vinyl chloride comprising polymerizing the corresponding monomer or monomers in microsuspension, in the presence of two seeding substances in the form of dispersions of polymer particles with a different average diameter, the particles of at least one of the seeding substances containing the initiator necessary for polymerization.
This process gives a latex containing two populations of particles with respective average diameters ranging from 0.4 to 2.5 .mu.m and from 0.08 to 1 .mu.m, in an average diameter ratio of from 1 to 20 and a weight ratio from 0.1 to 10.
The latex obtained by this process contains polymers which, after the separation of the polymerizing medium, are in the form of a powder particularly appropriate for the preparation of plastisols with a Newtonian or pseudoplastic action, containing very small quanties of plasticizer, down to 25 parts per 100 parts by weight of polymer, and with a low viscosity, both at a low speed gradient and a high one, thus allowing for very high speed coating.